This invention relates to a method of dehalogenating a halogen-containing hydrocarbon such as a halogenated ethane or propane and recovering a resultant halogen in elemental form separate from the dehalogenated hydrocarbon.
Most of dehalogenation reactions for organic compounds involve hydrogenation, but dehalogenation of some organic halides can be achieved without accompaniment of hydrogenation by the use of a metal dehalogenating agent as exemplified by the following reactions. ##STR1## The reactions of Equations (1) and (2) are known as the most popular laboratory methods of preparing olefins and alkynes. These reactions are applicable to polyhalogenated alkanes such as 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, pentachloroethane, hexachloroethane, 1,2-dibromoethane and 1,1,2-tribromoethane to obtain corresponding olefins. The reaction of Equation (3) is of use for preparing cycloparaffins in laboratories from 1,3-dichloropropane, 1,4-dichlorobutane, 1,4-dibromobutane, etc.
With respect to the dechlorination of 1,1,2-trichloro-1,2,2-trifluoroethane (Freon-113) to give chlorotrifluoroethylene by the use of zinc as represented by Equation (1), J. Am. Chem. Soc., Vol. 55, p. 2231 (1933) shows the use of alcohol as a dispersion medium, and U.S. Pat. No. 2,774,798 shows the use of an aqueous reaction system comprising a detergent together with zinc.
Also it is well known that, in certain cases, one molecule of a halogen can be separated from two molecules of a monohalogenated compound by the use of a metal dehalogenating agent: for example, biphenyl can be synthesized by debromination with copper powder of two molecules of bromobenzene.
The above described dehalogenation using a metal dehalogenating agent features a high selectivity of the product and in this regard is advantageous over other types of synthesis methods. However, this method is hardly appreciated as an industrially economical method because of the consumption of a dehalogenating agent in a quantity stoichiometrically corresponding to the quantity of the dehalogenated product. Besides, industrial application of this method is unfavorable from the viewpoint of preventing environmental pollution by reason of the formation of a noxious metal halide as an inevitable by-product.
As a different type of dehalogenation method for halogen-containing organic compounds, cathodic reduction of certain halogenated alkanes in formamide is reported in J. Org. Chem., Vol. 39, p. 3803 (1974) and Tetrahedron Lett. (1969), p. 1043. Presumably, this method will suffer from difficulty in separating the product from the reaction system and recovering the detached halogen.